Various theories of operation of high efficiency amplifiers have been developed over the years. However, microwave power amplifiers have only recently had maximum drain efficiencies exceeding 70 per cent. One approach in the related art has been utilizing phase shifted feedback. One of the obstacles to achieving high efficiency in amplifiers has been the nonlinearity of amplifier elements which have resulted in harmonics in the amplified signals. Particular management of the harmonics in the design of the amplifiers has an effect on the efficiency of the amplifiers.
There is a need for achieving high efficiency in amplifiers which are battery powered. Low losses are particularly critical in the high power circuits where the dissipated power not only drains the power source but often substantially increases the junction temperature of the device. High operational temperatures thereby lead to lower performance and lower reliability of the amplifiers.
A 100 per cent efficient amplifier can be achieved when the fundamental frequency at the output of the amplifier is terminated into a matched impedance. The odd harmonics are terminated into an open circuit and the even harmonics are short circuited. The concept is referred to as a class F amplifier. However, such scheme of obtaining high efficiency requires open and short circuits over many harmonics since the amplitudes of the higher harmonics may be rather substantial. For instance, the amplitude of the 9th harmonic of the ideal square waves is equal to 11 per cent of the fundamental frequency amplitude. Such wideband open and short circuit terminations with no losses and/or reflections for so many harmonics is impractical in the microwave frequency range.